75SeHCAT whole body retention for unexplained chronic diarrhoea based on early first whole body counting

There are many protocol variations in the whole-body 75SeHCAT retention test [whole-body retention (WBR)] for investigation of bile acid diarrhoea. The time between capsule consumption and first count, however, is widely taken, without debate, to be 3 h. In the Covid-19 era, it is desirable to limit the time patients spend in the department. We, therefore, questioned the need for a 3 h interval between capsule administration and the initial count.

Using an uncollimated gamma camera, whole-body counting was performed at 5, 30 and 180 min after capsule ingestion in 24 patients with chronic diarrhoea. Geometric mean was taken of counts acquired from posterior and anterior projections. WBR was expressed as the ratio of 7 day-to-initial whole-body counts (%) to give WBR5, WBR30 and WBR180. A small meal was given at 60 min after capsule ingestion.

There was a close correlation between WBR30 and WBR180 (y = 1.0x - 0.29%; r = 0.99). For WBR180 values of <15% (lower limit of normal), there was close agreement between WBR30 and WBR180 (bias 0.03%; precision 0.7%). WBR5 overestimated WBR180. However, cWBR5, obtained by multiplication of WBR5 by 0.75, also correlated closely with WBR180 (y = 1.2x - 4.5%; r = 0.97), and there was close agreement between cWBR5 and WBR180 for WBR180 values <15% (bias 0.08%, precision 1.3%).

The first whole-body count in the 75SeHCAT test can be undertaken at 30 min postcapsule without loss of accuracy, or even 5 min if only subnormal values are considered relevant. No food is required after capsule consumption.